1. Field of the Invention
The present invention relates to an erase mark detecting circuit and an optical information recording/reproducing apparatus in which the erase mark detecting circuit can be applied in a suitable manner. In the information recording/reproducing apparatus according to the invention, an erase mark for erasing an information data recorded on a data area of an information recording medium, such as an optical card and optical disc, is superimposed on the information data recorded area and the erase mark superimposed on the information data recorded area can be detected by the erase mark detecting circuit in a correct and stable manner.
2. Prior Art Statement
In an optical information recording/reproducing apparatus, in which a non-rewritable information recording medium, such as an optical disc and an optical card, is used, when a defect exists on the data area of the information recording medium an erase mark should be recorded on the data area having the defect, so as not to reproduce the information data recorded thereon. Additionally, for the purpose of keeping a secret, such erase mark is also recorded on the data area, which is no longer necessary.
In Japanese Patent Publication No. 63-3379, a conventional erase mark detecting circuit is disclosed. In the conventional circuit, the erase mark 2 is recorded on the data area, on which an information data has been already recorded with the aid of data pits 1, as shown in FIG. 1A, by superimposing a signal having a low frequency band on the data area. The erase mark 2 is detected when the information data recorded on the data area is reproduced in the optical information recording/reproducing apparatus, such that the information data reproducing signal is supplied to a frequency separating circuit 3, which comprises a low pass filter, etc, to derive a low frequency component of the information data reproducing signal. FIG. 1B is a schematic view showing the low frequency component of the information data reproducing signal. In the conventional circuit, the output of the frequency separating circuit 3 is supplied to a judging circuit 4 for judging an existence of the low frequency component in the output. FIG. 2 is a block diagram showing the conventional erase mark detecting circuit.
However, in the conventional erase mark detecting circuit, there is a problem that the erase mark is sometimes detected erroneously. The erroneous detection of the erase mark depends on the contents of the recorded information data or the recording method which was used in recording the information data on the data area. For instance, in the case that the information data is recorded in MFM modulating system with the aid of data pits, long and short distances between the data pits may be repeatedly formed on the data area with some frequency, as shown in FIG. 3A. However if the frequency of the long and short distances is similar to the frequency component of the erase mark, the same frequency component as that shown in FIG. 1B would be detected in the frequency separating circuit 3 from the information data reproducing signal; and the low frequency component would be supplied to the judging circuit 4. In the judging circuit 4, it is impossible to judge the low frequency component of the information data reproducing signal from that of the erase mark signal, so that the erase mark is erroneously detected from the data area on which the erase mark has not been recorded.